In many disciplines of science and engineering, very large and very small numerical quantities must be managed. Some of these quantities are mind-boggling in their size, either extremely small or extremely large.
Take for example the mass of a proton, one of the constituent particles of an atomās nucleus:
Proton mass = 0.00000000000000000000000167 grams
Or, consider the number of electrons passing by a point in a circuit every second with a steady electric current of 1 amp:
1 amp = 6,250,000,000,000,000,000 electrons per second
A lot of zeros, isnāt it? Obviously, it can get quite confusing to have to handle so many zero digits in numbers such as this, even with the help of calculators and computers.
Take note of those two numbers and of the relative sparsity of non-zero digits in them. For the mass of the proton, all we have is a ā167ā preceded by 23 zeros before the decimal point. For the number of electrons per second in 1 amp, we have ā625ā followed by 16 zeros. We call the span of non-zero digits (from first to last), plus any zero digits not merely used for placeholding, the āsignificant digitsā of any number.
The significant digits in a real-world measurement are typically reflective of the accuracy of that measurement. For example, if we were to say that a car weighs 3,000 pounds, we probably donāt mean that the car in question weighs exactly 3,000 pounds, but that weāve rounded its weight to a value more convenient to say and remember. That rounded figure of 3,000 has only one significant digit: the ā3ā in front ā the zeros merely serve as placeholders.
However, if we were to say that the car weighed 3,005 pounds, the fact that the weight is not rounded to the nearest thousand pounds tells us that the two zeros in the middle arenāt just placeholders, but that all four digits of the number ā3,005ā are significant to its representative accuracy. Thus, the number ā3,005ā is said to have four significant figures.
In like manner, numbers with many zero digits are not necessarily representative of a real-world quantity all the way to the decimal point. When this is known to be the case, such a number can be written in a kind of mathematical āshorthandā to make it easier to deal with. This āshorthandā is called scientific notation.
Scientific Notation
With scientific notation, a number is written by representing its significant digits as a quantity between 1 and 10 (or -1 and -10, for negative numbers), and the āplaceholderā zeros are accounted for by a power-of-ten multiplier. For example:
1 amp = 6,250,000,000,000,000,000 electrons per second
. . . can be expressed as . . .
1 amp = 6.25 x 1018 electrons per second
10 to the 18th power (1018) means 10 multiplied by itself 18 times, or a ā1ā followed by 18 zeros. Multiplied by 6.25, it looks like ā625ā followed by 16 zeros (take 6.25 and skip the decimal point 18 places to the right). The advantages of scientific notation are obvious: the number isnāt as unwieldy when written on paper, and the significant digits are plain to identify.
But what about very small numbers, like the mass of the proton in grams? We can still use scientific notation, except with a negative power-of-ten instead of a positive one, to shift the decimal point to the left instead of to the right:
Proton mass = 0.00000000000000000000000167 grams
. . . can be expressed as . . .
Proton mass = 1.67 x 10ā24 grams
10 to the -24th power (10ā24) means the inverse (1/x) of 10 multiplied by itself 24 times, or a ā1ā preceded by a decimal point and 23 zeros. Multiplied by 1.67, it looks like ā167ā preceded by a decimal point and 23 zeros. Just as in the case with the very large number, it is a lot easier for a human being to deal with this āshorthandā notation. As with the prior case, the significant digits in this quantity are clearly expressed.
Because the significant digits are represented āon their own,ā away from the power-of-ten multiplier, it is easy to show a level of precision even when the number looks round.
Example
Taking our 3,000 pound car example, we could express the rounded number of 3,000 in scientific notation as such:
car weight = 3 x 103 pounds
If the car actually weighed 3,005 pounds (accurate to the nearest pound) and we wanted to be able to express that full accuracy of measurement, the scientific notation figure could be written like this:
car weight = 3.005 x 103 pounds
However, what if the car actually did weigh 3,000 pounds, exactly (to the nearest pound)? If we were to write its weight in ānormalā form (3,000 lbs), it wouldnāt necessarily be clear that this number was indeed accurate to the nearest pound and not just rounded to the nearest thousand pounds, or to the nearest hundred pounds, or to the nearest ten pounds. Scientific notation, on the other hand, allows us to show that all four digits are significant with no misunderstanding:
car weight = 3.000 x 103 pounds
Since there would be no point in adding extra zeros to the right of the decimal point (placeholding zeros being unnecessary with scientific notation), we know those zeros must be significant to the precision of the figure.
